In recent years, with the development of imaging devices of higher densities, there is a remarkable trend toward higher resolution and downsizing of digital video cameras and digital still cameras. In an imaging device of a higher density, an area of a light-receiving element is smaller, so that long exposure time is required to ensure a Signal/Noise (S/N) ratio. When a position of a camera moves during exposure, image blurring occurs. To move the camera as little as possible, a photographer needs considerations such as fixing the camera to a tripod or, in the case of a handheld camera, holding the camera with the elbows brought tightly into the body.
Cameras provided with image blurring correction functions have been put to practical use, in order to reduce the burden of image blurring prevention on photographers. A fundamental concept of the image blurring correction functions is to detect a camera movement and obtain a captured image so as to compensate for the camera movement. Specific methods of compensating for the camera movement can be roughly classified into two types, namely, “electronic type” and “optical type”. The electronic type is a method of clipping a part of a captured image, whereby a clipping position is moved in an opposite direction to a camera movement to compensate for image blurring.
FIGS. 1A to 1C are explanatory views of an example of the electronic-type image blurring correction function.
As shown in FIG. 1A, a camera sets a clipping frame 11 which is smaller than a captured image 10. The camera outputs an image of the clipping frame 11 as an output image 12, which is displayed on a display screen or recorded on a recording medium. A captured image 13 shown in FIG. 1B is an image of a frame immediately following the captured image 10. A camera movement 14 occurs during a period from the image capturing in FIG. 1A to the image capturing in FIG. 1B. Without the image blurring correction function, even when a subject 15 is stationary, the position of the subject 15 in the screen changes as shown by an output image 16. This difference between the output image 12 and the output image 16 is “image blurring”. If both the camera and the subject are stationary, the position of the subject in the output image should be unchanged.
Accordingly, the camera having the image blurring correction function moves the clipping frame 11 by a camera movement correction vector 17 that has an opposite direction to and a same magnitude as the camera movement 14 as shown in FIG. 1C, thereby resolving image blurring and obtaining an output image 18 which is the same as the output image 12.
On the other hand, a camera having the optical-type image blurring correction function mechanically moves a lens system or an imaging device so as to compensate for the camera movement. That is, image blurring correction is performed directly on an image formed on the imaging device.
In electronic-type image blurring correction, the clipping frame 11 is moved in accordance with the camera movement, as explained with reference to FIGS. 1A to 1C. This requires accurate detection of the camera movement. The camera movement is detected on the basis of a movement of a captured image. When the subject is stationary, the accurate camera movement can be easily detected. In reality, however, there is a case where an image of a moving subject is captured. In particular, a video camera is mainly intended for recording a moving image by following a movement of a subject with time, and so there is hardly any instance where the subject is stationary.
In view of this, for example Patent References 1 to 3 disclose electronic-type image blurring correction techniques that can correct image blurring even in the case of capturing an image of a moving subject.
FIG. 2 is a block diagram showing a structure of a motion vector detection apparatus included in an image capturing apparatus of Patent Reference 1.
This motion vector detection apparatus includes a motion vector detection unit 31, a motion vector judgment unit 32, a divergence calculation unit 33, an LPF 34, a conversion unit 35, and a control unit 36.
The motion vector detection unit 31 detects a motion vector for each block of a captured image.
FIGS. 3A and 3B each show a motion vector detected for each block.
As shown in FIG. 3A, the motion vector detection unit 31 divides a captured image 20 into a plurality of blocks, and detects a motion vector for each block. In the case where a subject is stationary, motion vectors of all blocks are in agreement, and these motion vectors represent a camera movement. In the case where a moving subject is present, on the other hand, motion vectors of all blocks are not in agreement. In this case, the motion vector detection unit 31 detects motion vectors having different directions, as shown in FIG. 3B. The divergence calculation unit 33 calculates “divergence” indicating an extent of distribution of these motion vectors. The control unit 36 calculates an overall motion vector (camera movement) of the captured image 20 according to the divergence. In detail, the conversion unit 35 sets a relation between a motion vector coefficient for controlling a magnitude of the overall motion vector of the captured image and the divergence so that the magnitude of the motion vector decreases as the divergence increases.
FIG. 4 shows the relation between the motion vector coefficient and the divergence.
The conversion unit 35 decreases the motion vector coefficient of the motion vector when the divergence is larger, to reduce the effect of the motion vectors having larger divergence on the overall motion vector of the captured image. The control unit 36 controls the magnitude of the overall motion vector of the captured image on the basis of this processing result of the conversion unit 35.
Thus, even when the captured image contains a moving subject, the image capturing apparatus of Patent Reference 1 can correct image blurring by reducing the effect of motion vectors of the subject.
Meanwhile, Patent References 2 and 3 disclose techniques of detecting a camera movement by using only motion vectors in an outer part of a captured image, without using motion vectors in an entire area of the captured image. In detail, an image capturing apparatus of Patent Reference 2 detects a camera movement by using motion vectors in four corners of a captured image. Often a captured image contains a moving subject in its center part and a stationary background in its outer part such as the four corners. This being so, image capturing apparatuses of Patent References 2 and 3 detect a camera movement using only motion vectors in the outer part, and move the clipping frame according to the detected camera movement.
In this way, even when a captured image includes a moving subject, the image capturing apparatuses of Patent References 2 and 3 can correct image blurring by detecting a camera movement using only motion vectors in the outer part where the moving subject is not present.    Patent Reference 1: Japanese Patent No. 2506469 (FIGS. 1 and 2)    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 6-350895 (paragraph 0007, FIG. 2)    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 5-22645